[QE-users] info about setting celldm

[Pietro Davide Delugas]

Hello Patrizio
yes it is correct for F.C.C. lattice you need to specify ibrav=2. An 
celldm(1) is the cube's edge length.
As a rule the celldm and also the A,B,C and cosAB, cosBC, cosAC are 
always those of the "conventional" l cell. It means the cube for cubic 
lattices , the tetragonal paralleliped for the
tetragonal lattices and so on. See also the documentation  on ibrav 
https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm199
regards - Pietro

On 11/23/21 3:08 PM, patrizio.graziosi at cnr.it wrote:
>
> Hello everybody,
>
> I'd like to have from you a proof/disproof of my understanding on the 
> use of celldm for F space groups.
>
> I read in the instructions that QE uses the primitive cell, in the 
> case of Silicon I mean for that the "slanted" cell having parameter 
> around 0.387 nm and 60° angles, say like here 
> https://www.iue.tuwien.ac.at/phd/ungersboeck/node27.html .
>
> However, in an example for silicon, here 
> https://www.quantum-espresso.org/resources/tutorials/shanghai-2013/getting-started/lecture1.pdf 
> , I see that in the case of silicon with ibrav = 2 the entered celldm 
> correspond to 0.543 nm, the side of the "cubic cell".
>
> Thus, if I'm working with a F space group, ibrav = 2, I must use the 
> cube side as celldm, ~ 0.54 nm for silicon. Is this correct?
>
> If I'd like to work with the "slanted primitive cell" with angles of 
> 60°, which in silicon has lattice constant ~ 0.387 nm, how can I do? I 
> apologize but I couldn't find this in any tutorial I found.
>
> Thanks for your time,
> Patrizio
>